Field of the Invention
The present invention relates to a polishing apparatus.
Background Art
Conventionally, a polishing apparatus that uses a polishing pad, which is rotationally driven, as disclosed in WO 2013/038573, for example, has been known as a polishing apparatus that polishes a surface of a wafer made of a semiconductor material, and the like. In general, the polishing pad is provided at a distal end of a rotation shaft. The polishing pad including a rotation mechanism and a rotation shaft is supported by a head via an elastic mechanism. The head is supported by an apparatus main body via a driving mechanism.
It is possible to employ a method of combining position control and load control, as disclosed in JP S59-219152 A, for example, as a method of controlling a driving mechanism. This control method is a method in which the driving mechanism is controlled based on a position (coordinate) of a polishing pad (the position control) until achieving a target load when polishing is performed by the polishing pad in contact with a wafer, and the driving mechanism is controlled based on pressure caused on a contact surface between the polishing pad and the wafer (the load control) after achieving the target load. A position (Z coordinate) of a head when achieving the target load at the time of the polishing is determined based on the area of a polishing surface of the polishing pad and a spring constant value of an elastic mechanism.
It is possible to measure a load applied to the polishing pad using a load cell provided above the rotation shaft of the polishing pad, for example. The measurement procedure is as follows. That is, the head is lowered (moved in a direction of approaching the wafer) along a Z coordinate axis (vertical axis), and the polishing surface of the polishing pad is pushed against the wafer. Along with this, the elastic mechanism (coil spring) is deformed, and the polishing pad and the rotation shaft are relatively moved upward with respect to the head. Accordingly, an upper end portion of the rotation shaft is pushed to the load cell. Further, a strain gauge inside the load cell is deformed, and a force of the upper end portion of the rotation shaft pushing the strain gauge is measured. Further, a Z coordinate value of the head is adjusted such that the force to be measured by the load cell becomes the target load.
In the polishing apparatus described above, the polishing pad is rapidly moved upward (pushed up), and a measurement value of the load cell rapidly increases when a polishing body passes through a location where a tiny protrusion and a foreign substance is present at the time of performing the load control. In general, it is difficult to perform the highly accurate load control in accordance with such a rapid change of the load.
Further, there is a case where an overshoot with respect to the target load at the time of polishing is caused due to expansion and contraction of the elastic body when the position control is switched to the load control in such a polishing apparatus. When the overshoot is generated, a deviation occurs in a cut amount (polishing depth) of the wafer, and as a result, the accuracy in polishing of the wafer deteriorates. Examples of a technique of reducing the overshoot with respect to the target load in the polishing apparatus that performs the load control are disclosed in JP 2003-94328 A, JP 2003-326456 A, JP 2005-514780 A, JP 2007-181895 A, and JP 2015-35165 A. However, none of the documents has conducted studies on a method of reducing the overshoot caused by expansion and contraction of the elastic mechanism.